Conventionally, patent literature 1 discloses a technique for achieving normally-off and low on-resistance in a nitride semiconductor device including multiple channels. Specifically, a hetero-junction unit of an AlGaN layer and a GaN layer is repeatedly formed on the GaN layer to provide a natural super junction structure (hereinafter, referred to as NSJ structure). The nitride semiconductor device includes a first gate structure portion reaching the lowermost AlGaN layer of the NSJ structure and a second gate structure reaching the AlGaN layer above the lowermost AlGaN layer. The nitride semiconductor device includes a source region and a drain region formed of an n-type region at both sides of the first gate structure portion and the second gate structure portion.
In this nitride semiconductor device, each of the gate structure portions has a MOS structure. An electrostatic potential of a first gate electrode and a gate insulation film of the first gate structure portion is lower than a conductive band of the hetero-junction unit of the GaN layer and the AlGaN layer. Therefore, carries do not exist at a hetero-interface and normally-off operation is achieved. Since the nitride semiconductor device has multiple layers of hetero-junction units, a generation quantity of 2-dimensional electron gas (hereinafter, referred to as 2 DEG) is increased and an on-resistance is decreased. Also, a desired off-breakdown voltage is achieved by a polarization effect regardless of a stacked number of the hetero-junction units.